As the second most abundant membrane receptor complex on the platelet surface, the glycoprotein (GP)Ib-IX-V complex is essential to platelet physiology. Initially identified as the receptor for von Willebrand factor (VWF) and thus critical o hemostasis and thrombosis, GPIb-IX-V is also tapped for additional roles in inflammation, cancer metastasis, platelet genesis and clearance. Malfunction of this multi-subunit receptor complex can lead to severe bleeding diathesis and contribute to many cardiovascular diseases. However, it is not clear how this complex carries out its multi-faceted functions, even its canonical function to sense the elevated fluid shear stress in the blood vessel through its interaction with VWF. Relatedly, another fundamental question about GPIb-IX, the functions of Ib and IX subunits in the complex that do not directly participate in the interaction with VWF, has remained unanswered. Following up on our elucidation of the structure and quaternary organization of GPIb-IX-V, particularly our recent identification of a juxtamembrane mechano-sensitive structural domain (MSD) in the stalk region of Iba, we hypothesize that VWF binding and pulling on the N-terminal domain of Iba subunit induces unfolding of the MSD, which triggers a conformational change in the nearby Ib and IX extracellular domains. We propose to test this potentially paradigm-shifting hypothesis with a multidisciplinary approach in 3 specific aims. Aim 1 is characterize the biophysical nature and the functional consequence of force-induced unfolding of the Iba MSD. Aim 2 is to characterize the Iba MSD and test whether its intrinsic instability underlies the pathogenesis of Bernard-Soulier syndrome. Aim 3 is to determine whether and how a conformational change in the Ib and IX extracellular domains mediates transmembrane signaling of GPIb-IX. Completion of the proposed studies will help to elucidate the critical mechanosensing mechanism by the GPIb-IX-V complex, address the interplay between blood and flow, provide direction for future investigation of many unexplained phenomena about GPIb-IX-V, and devise novel therapeutic strategies to treat related bleeding diseases.